This invention relates to the communication of data, and more particularly to a method of encoding and decoding data to be transmitted serially along a communication network in an environmentally hostile environment, such as in an automobile.
In order to transmit digital data over a communication network, the data is encoded at the transmitter and decoded at the receiver. Two known forms of encoding which are typically used with transformer coupled communication lines are Manchester Encoding and Alternate Mark Inversion. Both require a clock recovery circuit that uses a local oscillator or phased locked loop. These circuits can be complex and, accordingly, may be undesirable due to high cost and reliability problems stemming from their complexity.
Alternate Mark Inversion coding ("AMI") produces alternate positive and negative level pulses, symmetrical around zero volts, when successive high bits (ones) occur in sequence. As a result, AMI is a three-level or ternary signal, where a one is represented by either a positive-going or a negative-going pulse in a signal interval, while a zero is represented by the absence of a pulse in a signal interval. There is no DC component in the transmitted signal, the amount of energy in the signal at low frequencies is small, and compared with unipolar signalling such as standard serial non-return-to-zero (NRZ) data, AMI has a substantial advantage in that it has much more immunity to crosstalk. Crosstalk immunity with AMI is typically on the order of 23 dB.
Manchester coding uses only two levels for binary data, instead of a three-level signal as in AMI. Manchester coding uses the phase of a square wave signal to indicate a one or a zero. A zero has an opposite phase waveform from a one. Every signalling interval in Manchester coding contains a zero crossing to provide a good reference for timing recovery. Every interval contains an equal amount of positive and negative level so that the DC component of the composite signal is cancelled out.
Another encoding/decoding scheme used in digital data transmission is known as pulse width modulation (PWM). In this technique, a series of uniform amplitude pulses are transmitted. The duration of the pulses is modulated by the data, so that a binary one or zero is distinguished by the width of the pulse. PWM provides very simple clock recovery, and a minimal use of analog circuits. However, the varying DC component in a PWM signal makes it unsuitable for transformer coupled systems.
It has now been recognized that in certain applications, such as the high electromagnetic noise environment present in automotive systems, transformer coupling of data to a communication network is advantageous. If transformers are used to couple equipment to a twisted pair network cable, and the transformers are constructed to optimize balance, several benefits can be obtained. These include the reduction of line signal radiation by flux cancellation, a reduction in susceptibility to both magnetic and electric field interference by common mode rejection, and elimination of differential ground currents in the network cable. In an automobile environment, the voltage signals produced by currents flowing through the vehicle chassis ground are seen as common mode signals that would be completely cancelled if the transformers were perfectly balanced. An important result is that load switching transients cannot use the network data cable as an antenna to radiate RF interference.
Commonly-owned, copending U.S. patent application Ser. No. 07/315,471 filed concurrently herewith and entitled "Communication Network" describes a data transmission network that uses transformer coupling. The disclosure in that application is incorporated herein by reference.
It would advantageous to provide a digital data encoding and decoding scheme which enjoys the advantages of both AMI and pulse width modulation, and is suitable for use in transformer coupled systems. The present invention provides such a system.